The present invention relates to a fuel supply system for engines and, more particularly, an improved fuel supply system for gasoline engines having a carburetor.
The gasoline engines for automobiles which incorporate an exhaust gas recombustion system such as a catalytic converter or a thermal reactor in order to meet the regulations with regard to engine emission generally cause a higher engine environment temperature which in turn is liable to cause an overheating of the fuel existing in the fuel supply system for the engine including the carburetor. If the temperature of the fuel exceeds 50.degree. C., it violently evaporates, thereby generating a lot of bubbles in the fuel supply system, resulting in obstruction of the correct control of air/fuel ratios, unstable rotation of the engine, and an increase in the emission of harmful gas components.
In normal operation of automobiles, the fuel supply system means such as the carburetor and other fuel supply passage means are provided with a cooling effect by the flow of intake air and the heat-absorbing evaporation of fuel. However, when the engine is brought to idling operation or stop subsequent to high-speed high-load operation, the fuel supply system is very liable to overheat. If the fuel existing in the fuel supply system is overheated, the viscosity of fuel greatly lowers and causes an unacceptable change of the flow coefficient of the jet means incorporated in the carburetor. Furthermore, the fuel existing in the float chamber and other passages of the carburetor causes boiling and generates a lot of bubbles. These bubbles carry fuel droplets and take them into the air passage of the carburetor and so-called percolation occurs. If percolation occurs, the carburetor generates an over-rich fuel-air mixture which causes unstable operation of the engine, an increase of emission of uncombusted harmful components and poor restartability of the engine.
Furthermore, if the fuel bubbles are generated in the carburetor or other fuel supply passage means, they interrupt the flow of fuel therethrough and cause so-called vapor-locking. If vapor-locking occurs at any part of the fuel supply passage means, the supply of fuel is impeded and in the worst case the engine stops. Even when the engine does not stop, misfiring is caused. It is known that the abovementioned percolation or vapor-locking is caused principally by light components having low boiling points included in gasoline.
In order to avoid percolation or vapor-locking, it has been proposed, and practiced, to feed an excessive amount of fuel from the fuel tank to the carburetor by employing a fuel pump having a large excess capacity, so that a substantial flow of fuel is always maintained in the fuel pump and the fuel supply passage means in order to avoid any undesirable temperature rise of the fuel supply system. This system is generally called a fuel return system, and is effective for reducing the occurence of percolation or vapor-locking. However, this system is not yet perfectly free from percolation or vapor-locking.